There are crafts and coloring pages throughout the site for nearly any birthday party theme imaginable.  You can use the site's Search feature if you're having difficulty locating something.  

Many of the theme sections include a "Birthday Ideas" section which includes suggestions for decorations, treat bag items and games!

I've listed a few of the themes I've received specific party requests for or that seem particularly popular.

Birthday Themed Paper Crafts:
(these extra large paper crafts make great wall hangings/decorations for the birthday)

Discussions we've had on the site about Birthday Parties. 

Go to Discussion Group to see discussions on many birthday themes.  I am afraid the discussion group is no longer active but the old posts are still available to provide you with inspiration.